U.S. Pat. No. 5,469,541 describes an example of overlaying by window specific control of overlay planes in a graphics display system. By a graphics environment window the characteristics of an overlay common to multiple-windows are controlled while operating within the context of a conventional RAMDAC overlay control architecture. Window specific overlay control is accomplished by concatenating the window, masking and overlay data as an address to a mapping memory. The bit content of the mapping memory is controlled directly by the general purpose processor to selectively refine the relationship between the concatenated input as an address and the mapping memory output as the state conveyed to the overlay control of the RAMDAC. A common overlay is thus selectively modifiable by window
Overlaying planes or windows requires respective image data to be fetched from memory. The known system requires many memory fetches to retrieve the pixel values for the layers to be overlayed.